Field of the Disclosure
The present application relates to a foldable display device. More particularly, the present application relates to a foldable display device developed to minimize the generation of surface waviness which is caused by repeated fold and unfold operations.
Description of the Related Art
Nowadays, public demands for applying a large-sized screen in portable devices, such as mobile phones, tablet personal computers and so on, are being increased. As such, the general public shows a keen interest in foldable display devices. The foldable display devices have features of easier portability, a larger-sized screen and so on.
The foldable display devices developed up to the present can cause waviness in a fold portion by being repeatedly folded and unfolded. As shown in FIG. 1, a foldable display device of the related art includes a touch panel 40 disposed on a flexible display panel 30 and a cover window member 50 disposed on the touch panel 40. The touch panel 40 can realize a touch input. The cover window member 50 can prevent external impacts and scratches. Also, the related art foldable display device includes a back plate 20 disposed on a lower surface of the flexible display panel 30 configured to support the flexible display panel 30. A cover window 51 included in the cover window member 50 is combined with the touch panel 40 by means of an optical clear adhesive layer 52. The back plate 20 is combined with the flexible display panel 30 by means of another optical clear adhesive layer 25. Although it is not displayed in the drawing, a circular polarizing plate can further be disposed on one of upper surfaces of the display panel 30 and the touch panel 40. The circular polarizing plate can reduce the reflection of external light.
Such a related art display device allows several film layers or panels to be attached to one another using adhesion layers. As such, when the display device is folded, a contractile force is applied to the film, panel and adhesive layer disposed in an upper portion of the display device and a tensile force is applied to the film, panel and adhesion layer disposed in a lower portion of the display device. The compressive and tensile forces can cause a plastic deformation of the film and a deformation of the adhesion layer. Due to this, the waviness can be generated in a fold portion of the display device which is largely affected by the contractile and tensile forces. Particularly, if the display device has been folded for a long time or repeatedly, the waviness corresponding to an outside appearance defect can be generated because a waved surface of the display device cannot be restored. The waved portion of the surface of the display device can deteriorate a degree of touch sensitivity. Due to this, the touch panel cannot be normally driven.
FIG. 2 is a photograph showing the related art foldable display device which is unfolded after it has been folded in a curvature of about 2.5 R for 24 hours. As seen from FIG. 2, it is evident that the waviness is generated in the surface of the foldable display device.